As-Salaam alaikum,
One of the companions of the Holy Prophet, Sallallahu alaihi Wasallam, that is Abdur Rahman ibn Awf, may Allah be pleased with him, reported that the Messenger of Allah, Sallallahu alaihi Wasallam, said :-
"VERILY, RAMADAN IS A MONTH IN WHICH ALMIGHTY ALLAH HAS OBLIGATED ITS FASTING. I HAVE INSTITUTED FOR MUSLIMS THE PRACTICE OF PRAYER AT NIGHT. THUS, WHOEVER FASTS IT (Ramadan) WITH FAITH AND EXPECTING REWARD, WILL BE RID OF SINS LIKE THE DAY HE WAS BORN FROM HIS MOTHER. "
(Musnad Ahmad, 1688. Graded as Sahih).